Test command to visualize printf output from the target device, term using DCC (SEGGER DCC handler running on target)

CoreSight register

ReadAP
ReadDP
First read returns the data of the previous read. An additional read of DP reg 3 is necessary to get the data.
For SWD, data is returned immediately.
For JTAG the data of the previous read is returned.
An additional read of DP reg 3 is necessary to get the data.

VTREF ValuemV
15.4 Reference voltage (VTref)
VTref is the target reference voltage. used by the J-Link to check if the target has power, to create the logic-level reference for the input comparators and to control the output logic levels to the target. It is normally fed from Vdd of the target board and must not have a series resistor.
In cases where the VTref signal should not be wired to save one more pin / place on the target hardware interface connector (e.g. in production environments), SEGGER offers a special adapter called J-Link Supply Adapter which can be used for such purposes. Further information regarding this, can be found on the SEGGER website ( J-Link supply adapter )
To guarantee proper debug functionality, connect at least on of the GND pins to GND (Pin 4, 6, 8, 10, 12, 14*, 16*, 18*, 20*).
On some models like the J-Link ULTRA, these pins are reserved for firmware extension purposes. They can be left open or connected to GND in normal debug environ- ment. Please do not assume them to be connected to GND inside J-Link.